1. Field of the Invention
The present invention relates to a rotary printing machine with a form cylinder and another cylinder each having a gearwheel. The form cylinder is movable between an on position and an off position. The gearwheel of the other cylinder engages the gearwheel of the form cylinder when the form cylinder is in the on position so that the other cylinder drives the form cylinder. The rotary printing machine also includes an imaging device for imaging the form cylinder while the form cylinder is in the off position. The gearwheels are either partially engaged or disengaged when the form cylinder is in the off position.
2. Description of the Related Art
German reference DE 43 03 872 C2 discloses a prior art rotary printing machine with a form cylinder that can be imaged within the rotary printing machine such, for example, as by a laser writing system or a thermal writing system.
In printing machines which are driven by gearwheels via a wheel train, imaging is performed on the form cylinder when the form cylinder is pivoted or otherwise positioned away from the transfer cylinder to all off position and the transfer cylinder is also pivoted or moved away from a back-pressure cylinder to an off position. The pivoting results in a gap, the order of magnitude of which is a few tenths of a millimeter or several millimeters, that is large enough so that the form cylinder and transfer cylinder and the transfer cylinder and back-pressure cylinder do not touch one another. The transfer cylinder and back-pressure cylinder must also no longer touch the printing material in the off position. In embodiments which do not include a transfer cylinder, the form cylinder is driven by the back-pressure cylinder and the back-pressure cylinder must not touch the form cylinder or the printing material when the form cylinder is in the off position.
When the form cylinder is in the on position, the necessary pressure force between the form cylinder and transfer cylinder is achieved by a rubber coating of the transfer cylinder (rubber blanket cylinder). When the form cylinder is in the on position on the back-pressure cylinder, the latter must carry a corresponding rubber coating to achieve the necessary pressure force. To achieve sufficient pressure force, the form cylinder must be pressed against the other cylinder so firmly together that a slight deformation of the cylinders occurs in the region of the contact zone. Cylinder interference is of the order of magnitude of 0.2 mm. Therefore, to achieve sufficient clearance between the cylinders when the form cylinder is moved to the off position, the form cylinder is required to move a distance of 0.2 mm plus a few tenths of a millimeter as a safety reserve.
For an optimum printing cycle, it is necessary for the gearwheels, which are arranged on a lateral side of the cylinder bodies on the form cylinder, transfer cylinder or back-pressure cylinder, to roll optimally one on the other in a fully meshed engagement when the cylinders are in the on position. When the cylinders are moved to the off position, the gearwheels are not in full meshed engagement and have increased tooth play, thereby causing the gearwheels to roll unfavorably one on the other insofar as the gearwheels still touch one another. If the gearwheels are fully disengaged when the cylinders are in the off position, the top portions of the teeth of the two gearwheels may abut one another during an attempt to return to the on position which may prevent the cylinders from fully moving to the on position or cause damage to the gearwheels. The form cylinder is normally required to be in the off position during imaging. Because of the uncontrolled movement cycles of the two adjacent gearwheels in relation to one another caused by the tooth play when the cylinders are in the off position, and because of the unfavorable rolling of the toothed flanks when the gearwheels still touch one another, rotational speed wobbles occur when the form cylinder is in the off position which impair the image quality when the form cylinder is imaged within the printing machine. Imaging operations are particularly sensitive to rotational speed fluctuations when the rotational speed fluctuations are of higher frequency such, for example, as have a frequency of more than ten Hz. The control of the imaging device cannot follow such rapid rotational speed fluctuations. At a typical circumferential speed such for example, as 0.5 m/s, these fluctuation frequencies may lead to streaking in spatial frequency ranges in which the human eye is particularly sensitive. With the circumferential speed of 0.5 m/s and a rotational speed fluctuation with a frequency of 100 Hz, the resultant imaging fluctuations would occur in a region of 5 mm in the direction of printing which would be easily detectable. Thus, even slight density fluctuations, for example of the order of magnitude of 1%, which are caused by the rotational speed fluctuations, may lead to visible streaking. In contrast, low frequency rotational speed wobbles are not critical because the imaging control can easily follow slow changes in rotational speed and density fluctuations with low spatial frequencies such, for example, as those which occur within a length of several centimeters cannot be easily detected by the human eye.
It is already known from German reference DE 28 35 960 B1 to damp oscillations which may occur in the gearwheel drive mechanism of rotary printing machines. In the drive of the impression cylinders in this reference, at least two gearwheels forming a first power transmitting drive train are connected to a second power transmitting drive train in parallel using a belt. The belt is a flat or vee belt which is capable of slipping and is therefore not a sufficient means for eliminating the undesirable tooth flank play when the cylinders are in the off position.